In communications networks, the topology is usually a schematic description of the arrangement of a network, including its nodes and connecting lines. There are two ways of defining the network geometry: a physical topology and a logical topology. The physical topology of a network is the actual geometric layout of the nodes, while the logical topology refers to the nature of the paths that signals follow from node to node. In many instances, the logical topology is the same as the physical topology.
There are several common physical topologies, including the bus network, the star network, and the ring network. In the bus network topology, every node is connected to a main cable called the bus. Therefore, in effect, each node is directly connected to every other node in the network. In the star network topology, there is a central computer or server to which all the nodes are directly connected. Thus, every node is indirectly connected to every other node through the central computer. In the ring network topology, the nodes are connected in a closed loop configuration. Adjacent pairs of nodes are directly connected. Other pairs of nodes are indirectly connected, the data passing through one or more intermediate nodes.
The common goal for each of these physical configurations is a fully-connected topology, meaning each node is directly or indirectly connected to all other nodes in the network. To achieve a fully-connected topology, connections between nodes are made randomly, or in rare cases, based on the master/slave role of the nodes, rather than the data flow pattern among the nodes. This results in an inefficient communication network for a number of reasons. Nodes having similar profiles, indicating the likelihood of frequent communication with each other, are often connected through paths requiring multiple hops. At the same time, nodes having dissimilar profiles, indicating the improbability of frequent communication with each other, may be directly connected. Moreover, nodes may be connected, either directly or indirectly, where no connection is required because these nodes will never communicate with each other.